This invention relates generally to optical disks and more specifically to an optical disk that includes physical features suitable for use in detecting radial tilt of the disk relative to an ideal plane.
Optical disks, for example, compact disks (CD), require precise focusing of an optical beam onto a data surface. One or more light beams (typically from a laser diode), illuminate one or more spots on the disk, and are reflected back into an optical head. In addition to information about recorded data, the reflected light beams generally may also carry information about tracking error (how well a beam is centered on a data track), and focus error (how well a beam is focused onto the data surface). Typically, the data surface of an optical disk is protected by a transparent substrate on the side that is illuminated by the laser. In general, whenever an illuminating beam must pass through the substrate to reach the data surface, disk tilt degrades the focus quality of the illuminating beam. Typically, for the data densities involved in CD media, tilt detection and compensation are not required. However, for higher data densities, for example, for Digital Versatile Disks (DVD), radial tilt detection and compensation may be necessary. Note that tilt may have a radial component and a tangential component, but the radial component is typically much larger (and therefore of more concern) than the tangential component.
Some optical disk drives use a separate light beam for radial tilt measurement. See, for example, U.S. Pat. Nos. 5,657,303 and 5,699,340. In order to simplify the optical head, there is a need for a radial tilt measurement system that uses the same light beam that is used for reading the data. However, the tilt information should not interfere with the resulting data signal. One approach to providing radial tilt information in the data reading beam is used by the Advanced Storage Magneto Optic (ASMO) format. ASMO media is preformatted with permanent (embossed) headers. Each header starts with tilt measurement marks, formed into the walls of a groove defining a track. The tilt measurement information does not interfere with the data because data and headers do not coexist at the same place on the disk. However, rewritable DVD media do not use permanent headers. There is a need for radial tilt detection features, in optical media that do not use permanent headers, that will not interfere with the optical data signal. Another approach is to provide multiple tracking error signals. See, for example, U.S. Pat. No. 5,808,985. There is need for radial tilt detection without requiring any modification to conventional optical heads.
An optical disc medium in accordance with the invention has a recording thin film structure (data surface) with grooves and lands behind a transparent substrate. Periodically, along radial lines, radial tilt measurement features are provided in the data surface, wherein the height of the grooves and lands are changed, preferably over a short circumferential length. For example, along the radial lines, the height of a groove (over a short length) may be raised to the height of a land and the height of a land (over a short length) may be reduced to the height of a groove. The optical disk medium is designed in conjunction with the optical system of the drive so that when the focused laser spot is centered on a groove, a tracking error signal (for example, radial push-pull signal) is zero even if the disk is radially tilted. If the focused laser spot is centered on an area having a height that is different than the height of a groove, for example a land, the tracking error signal varies when the disk is radially tilted. As a result, when the focused laser spot passes over a tilt measurement feature, an abrupt step in the tracking error signal provides a measure of the magnitude and direction of radial tilt. The abrupt steps are removed from the tracking error signal by existing low-pass filtering. The tilt measurement features have negligible effect on the data signal, and negligible effect on filtered tracking error and focus error signals. No changes are required for the drive optical system. The only drive change required is additional signal processing of the tracking error signal to detect abrupt steps.
Some proposed formats (for example, DVD-RAM) use a format called single-spiral groove and land recording, in which each spiral groove completes one revolution of the disk and then ends at the beginning of a spiral land. Each spiral land completes one revolution of the disk and then ends at the beginning of a spiral groove. Data is recorded on the grooves and on the lands. The method of using a change in the groove height to measure radial tilt is also applicable to single-spiral groove and land recording.